It is known that a body transmitting or reflecting a light radiation can introduce variations in the state of polarization of the light radiation. Knowledge of the state of polarization of the radiation emerging from the body is of importance to completely characterize the body in respect of its optical properties, and is essential when exploiting interference or beats between radiations, since these phenomena occur only when the radiations are equally or correspondingly polarized.
Applications include well-known applications of classical optics, optical coherent or heterodyne telecommunications (based on beats) and optical fiber sensors or gyroscopes, requiring the use of fibers maintaining a determined state of polarization.
A polarized radiation can be characterized by electromagnetic field components in a reference system with orthogonal axes x, y. Considering the electrical field alone, the two components are given by: EQU Ex=a1 cos .omega.t Ey=a2 cos(.omega.t+.psi.) (1)
where a1, a2 are the amplitudes of the two components and .psi. is the relative phase. To determine the state of polarization it is necessary to measure the ratio a2/a1 between the two amplitudes and phase .psi., whose sign defines the rotation direction on the polarization image, described on plane Ex, Ey as t varies. From these two values further information can be derived necessary to characterize the body under test, e.g. polarization beat length, in case of single-mode optical fibers.
It is also to be noted that the state of polarization can vary in time. In case of optical waveguides, this usually occurs owing to variable mechanical and thermal stresses which modify their optical properties.
In order to determine time-varying polarization state, interferometric techniques have also proved to be useful. An example based on a Mach-Zehnder interferometer has been described by us in the article A heterodyne Mach-Zehnder polarimeter for real-time polarization measurement, Optics Communications, Vol. 54, No. 2, 15 May 1985, and in the paper "A fast heterodyne interferometer for real-time fiber polarimetry" presented at IOOC-ECOC '85, Venice, October 1985.
Yet this solution requires all the light beams inside the device to be coplanar, to avoid systematic errors which depend also on the polarization state to be determined and which hence cannot be eliminated by a simple instrument calibration.
A Michelson interferometer is intrinsically free from these disadvantages, since the light beam emitted from the source is split into two beams which are back-reflected; such beams are obviously coplanar, and the distance between the beam-splitter and the mirrors can be maintained very short.
An example of apparatus for measuring the state of polarization, based on a Michelson interferometer, is described in Ellipsometry and polarized light, by R.m.A. Azzam and N. M. Bashara, North-Holland Publishing Company, 1977, pages 262-265, and in the paper Automated laser interferometric ellipsometry and precision reflectometry, by H. F. Hazebroek and W. M. Visser, Journal of Physics, Section E, Vol. 16, 1983, pages 654-661.
These documents disclose an ellipsometer, i.e. a device for measuring the polarization state of radiation reflected by the surface of a body. In that ellipsometer, polarized radiation is split by a beam splitter into two fractions. One fraction is sent towards the body under test and reflected onto a mirror by which it is reflected back onto the body and hence to the splitter; the other, acting as a reference beam, is sent to a corner reflector and therefrom to the splitter. The corner reflector is oscillated so as to change by Doppler effect the frequency of the beam sent back towards the splitter in the reference branch. The two beams are recombined by the splitter into a single beam containing both frequencies. The components parallel and perpendicular to the incidence plane on the body under test are separated and sent to different detectors. A microprocessor obtains the required information from the intensities of the beat signals supplied by the detectors.
A system of this kind has a number of disadvantages. More particularly, the corner reflector position is critical, since it has to be chosen so as to make reference beam coincide with one of the two reflector self-polarizations, in order to maintain the reference beam polarization; there are moving parts, which always entail reliability problems; the system operates at low frequency (200 Hz) so that it does not allow detection of polarization fluctuations which are very rapid.